The present invention relates to the xerographic arts. The invention finds particular application in controlling operation of xerographic machines and will be described with particular reference thereto.
The prior art has taught the production of copies from document originals produced by the xerographic process wherein the document original to be copied is placed on a transparent platen, either by hand or automatically through the use of a document handler. The document original is illuminated by a relatively high intensity light. Image rays reflected from the illuminated document original are focused by a suitable optical system onto a previously charged photoconductive layer of a photoreceptor. The image light rays function to discharge the photoconductive layer in accordance with the image content of the original to produce a latent electrostatic image of the original document on the photoconductive layer. The latent electrostatic image so produced is thereafter developed by a suitable developer material such as toner, and the developed image is transferred to a sheet of copy paper brought forward by a suitable feeder. The transferred image is thereafter fixed as by fusing to provide a permanent copy while the photoconductive layer is cleaned of residual developer preparatory to recharging.
A photoreceptor is at the heart of the xerographic process. During charging a photoreceptor must be able to receive and hold a charge in the dark. During exposure, the photoreceptor must release that charge from areas exposed to the light. To accomplish this the photoreceptor incorporates photoconductive material.
In addition, the photoreceptor must be constructed so that the movement of electrons can be controlled. In other words, the photoreceptor must be constructed so that charges can be placed, held, and released at different times and under different conditions. To accomplish the control of the electron movement, the photoreceptor usually also includes a substrate layer.
The substrate has four major purposes. Three purposes have a strong effect in the charging, exposing, and cleaning processes and require a substrate material which is a good conductor. First, the substrate helps to maintain a uniform charge across the surface of the photoreceptor. Second, the substrate helps to control the field strength of the photoreceptor charge. Third, the substrate provides the electrical grounding for the photoreceptor. The fourth purpose for the substrate is physical in that it acts as a base for the very thin photoconductive layer.
The substrate of most photoreceptors is made of aluminum. Aluminum is a good conductor and it is also less expensive to refine, machine, burnish, and clean than most other conductors.
It is through the photoconductive layer that charges move, based on the presence of light. Several different materials are currently used for the photoconductive layer, such as a variety of organic compounds, selenium alloys, arsenic triselenide, cadmium sulfide, or amorphous silicon. The most common of these are organic compounds and selenium alloys.
Organic compounds, by definition, are chemical compounds based on carbon. FIG. 1 discloses a typical photoreceptor which has a substrate 12 and a organic photoconductive layer 14 which has two-layer construction; a charge generation layer 16 and a charge transport layer 18. The layer closest to the substrate is the charge generation layer 16. This layer contains the charges which move when the photoreceptor is acting as a conductor. When the photoreceptor 10 is charged, the induced charges are in the charge transport layer 16. Organic photoreceptors have an additional barrier between the photoconductive layer and the substrate. This barrier, commonly called the underlayer 20, prevents the easy flow of electrons between the substrate and the upper layers.
Photoreceptors can be damaged by chemicals, such as lubricants, fusing agents, the oils on fingers, by heat, or simply by the constant exposure to paper and developing agents. This damage translates into copy or print quality defects. Photoreceptor defects can range from scratches or abrasions in the photoconductive layer to the development of a film on the surface, oxidation, or rapid crystallization.
Another common manner in which photoreceptors tend to have their effective life expectancies shortened occurs by excessive charge-erase cycles where no images are actually exposed upon the charged photoreceptor. These "wasted" exposures on the photoreceptor greatly reduce the effective life of the photoconductive surface by increasing the wear and tear on the photoconductive film. In order to reduce these wasted exposures existing xerographic machines, especially those employing organic photoconductive layers, are designed so their control systems terminate all xerographic bias potentials and erase functions as soon as is practical after a last scheduled image has been reproduced.
In the majority of situations, especially those which occur on high productivity machines which employ fully automatic document handlers, this rapid xerographic shutdown prior to the actual machine shutdown will be transparent and non-invasive since no manual operator intervention is involved between produced images. However, this rapid xerographic cycle-down tends to greatly limit operator productivity when xerographic machines are employed to reproduce copies of individual manually or operator assisted semi-automatically positioned originals. This is true since during the time when the operator is manually exchanging originals, the xerographic subsystem control may cycle-down, resulting in a long "restart" time since it becomes necessary to re-enable the xerographic subsystem for several image zones prior to actual image production due to the physical dimensions of the xerographic system. Additionally, if the time between operations extends even further, the control subsystem for the entire machine may cycle-down resulting in an even longer original-to-original copy time.
Even very proficient operators often encounter long restart times due the conservative nature of existing xerographic subsystem designs. This problem is especially bad where the platen/glass cover needs to be closed over for each original prior to copying to avoid background or operator annoyance over the illumination intensity.
The present invention contemplates a new and improved control system for a xerographic imaging device. The xerographic imaging device includes an adaptive control system to allow for a more equitable compromise between manual placement job productivity and photoreceptor life.